The costs of electronic data storage have been dramatically reduced as the storage densities on recording media have increased. This trend is particularly evident in hard disk drive technology. A conventional magnetic recording disk 10 for use in hard disk drives is depicted in FIG. 1. A cross section A of FIG. 1 is enlarged and depicted schematically in FIG. 2A. A typical recording disk 10 includes an aluminum (Al) substrate 12 covered by a nickel phosphorous (NiP) plating 14. A chromium (Cr) underlayer 16 is provided on the nickel phosphorous plating. A magnetic material such as nickel, cobalt (Co), or a magnetic alloy is electroplated or sputtered onto the chromium underlayer 16 to form a continuous magnetic layer 18. A carbon overcoat 20 is deposited on top of the magnetic layer 18 and serves to protect the magnetic layer 18.
An enlarged top view of section B of the recording disk 10 of FIG. 1 is depicted in FIG. 2B. This view is not a physical view, but rather one provided by a magnetic force microscope following writing of data onto the disk 10. As recorded by a write head, the bits are substantially rectangular in shape and arranged in concentric tracks. In the illustrated example, a track width is approximately 2,000 nm. A small separation exists between the bits within a track as well as between bits of adjacent tracks.
The approximate dimensions of a conventional magnetic longitudinally recorded bit are as follows. The length of the bit is approximately 2,000 nm, the width of the bit is approximately 150 nm, and the depth of the bit is approximately 15 nm. A magnetic disk 10 that has been formed with a continuous magnetic layer 18 as depicted in FIG. 2A with the bit size described above has a recording density of approximately 1.7 Gbit/in.sup.2.
Increases in the areal density of magnetic storage media have been driven by the downward resealing of hard drive assemblies. This resealing includes reducing the size of the grains making up the magnetic layer. In longitudinal recording, each bit is composed of numerous grains in order to maintain an adequate signal-to-noise ratio. However, reducing the grain size in order to reach higher storage densities is limited by the superparamagnetic limit. This limit occurs at the grain size at which thermal energy alone can trigger random magnetic switching of the grains.
A technology has been proposed to greatly increase the recording density of a magnetic disk by using prefabricated discrete, single-domain magnetic elements embedded in a non-magnetic material. As proposed in Ultra High-Density Recording Storing Data in Nanostructures, Stephen Chou, Data Storage, September/October 1995 (pages 35-40), thin-film magnetic media is replaced by media that includes discrete magnetic elements embedded in a non-magnetic disk by an electron beam. A corresponding cross-section A is depicted in FIG. 3A for a magnetic disk 10 having the proposed quantum magnetic structure. A silicon substrate 30 is covered by a plating base 32. A silicon dioxide layer 34 is provided on the plating base. The silicon dioxide forms a non-magnetic isolation layer in which magnetic columns are provided. The non-magnetic layer 34 has a depth of approximately 100 nm. Magnetic columns 38, approximately 20 nm in diameter, are provided in a vertical orientation in the non-magnetic layer 34. The magnetic columns 38 may be made of nickel or cobalt for example. The non-magnetic layer 34 and the magnetic columns 38 are protected by an overcoat layer 36.
A schematic top sectional view of the proposed quantum magnetic disk is depicted in FIG. 3B, without the overcoat layer 36, to illustrate the arrangement of magnetic columns 38. In contrast to the magnetic force view of FIG. 2E, the view in FIG. 3B is a physical view. The centers of the magnetic columns 38 are separated by a distance of approximately 100 nm and are arranged in a grid-like manner. Each of the magnetic columns 38 represents a single bit for magnetic recording. The size of the bits (approximately 20 nm diameter) and the separation of the columns (approximately 100 nm) produces a recording density of approximately 65 Gbit/in.sup.2.
The costs associated with achieving such a large storage density are prohibitive as the proposed manufacture of quantum magnetic disks utilizes expensive semiconductor processing techniques. An exemplary fabrication process was described by Chuo as including electron beam lithography to define the size and location of each bit in the disk. After development and chrome etching, a reactive ion etching step is performed to create a silicon dioxide template with column openings. Nickel or another electromagnetic material is then electroplated into the column openings to form the magnetic columns. The disk is then polished to planarize its surface.
In addition to the greatly increased costs of manufacture of the disks, the proposed quantum magnetic disk requires complicated non-Winchester recording technology not currently available. Hence, although providing a very high recording density, the proposed quantum magnetic disk remains an impractical alternative to conventional magnetic recording media.
A technique for texturing a disk is disclosed in U.S. Pat. No. 5,062,021 in which a laser light beam is focused on a upper surface of non-magnetic substrate. The disclosed method comprises polishing an NiP plated Al substrate to a specular finish, and then rotating the disk while directing pulsed laser energy over a limited portion of the radius, to provide a textured landing zone while leaving the data zone specular. The landing zone comprises a plurality of individual laser spots characterized by a central depression surrounded by a substantially circular raised rim.
Another laser texturing technique is reported by Baumgart et al. with "A New Laser Texturing Technique For High Performance Magnetic Disk Drives," IEEE Transactions On Magnetics, vol. 31, no. 6, pp. 2946-2951, November 1995. See, also, U.S. Pat. Nos. 5,550,696 and 5,595,791.
In copending application Ser. No. 08/666,374 filed on Jun. 27, 1996, a laser texturing technique is disclosed employing a multiple lens focusing system for improved control of the resulting topographical texture. In copending application Ser. No. 08/647,407 filed on May 9, 1996, a laser texturing technique is disclosed wherein a pulsed, focused light beam is passed through a crystal material to control the spacing between resulting protrusions.
As areal recording density increases, the flying height must be reduced accordingly, thereby challenging the limitations of conventional laser texturing technology for uniformity and precision in texturing a landing zone to form protrusions. The requirements for continuous alignment and adjustment of the laser beam are exacerbated in geographical locations with relatively unstable environmental conditions, such as temperature, vibration and shock, particularly regions susceptible to seismological disturbances such as tremors and earthquakes. Conventional laser delivery systems for texturing a landing zone comprises a system of mirrors and lenses which must be precisely and accurately maintained, particularly as the flying height is reduced to a level of less than about 300 .ANG., due to inherent undulations of the substrate surface.
The above described limitations on conventional laser delivery systems has not allowed these systems to be used to form micro-to-nano machining of disk surfaces to create consistently reproducible features below approximately 200 nanometers in diameter. In copending application Ser. No. 60/037,627 filed on Jan. 15, 1997, now abandoned a laser texturing technique is disclosed that employs a fiber laser to provide a textured surface on a substrate. The laser light beam source of the system is optically linked to one end of a fiber optic cable. The other end of the fiber optic cable is optically linked to a lens, such as a micro-focusing lens. The micro-focusing lens is then positioned near a surface of a substrate for a magnetic recording medium and maintained in a fixed position, as with conventional clamps. The laser light beam impinges upon the rotating substrate. The clamping of the fiber laser in a fixed position in relation to the surface being textured prevents a constant optical focusing distance from being obtained if the desired optical focusing distance is very small. This is because the position of the output of the laser is fixed, but the substrate surface has inherent undulations that change the optical focusing distance from the laser output. The effect of the surface undulations on the optical focusing distance is not critical unless a substantially consistent optical focusing distance that is very small is required, for example to perform micro-to-nano machining.
There exists a need for a laser machining system which attains a consistent optical focusing distance, even when the distance is very small, while taking into account the inherent undulations of the surface to be machined.